1. Field of the Invention
The present invention relates to a nozzle protection device to be used for protecting a nozzle, which injects a target material, from plasma in a laser produced plasma type extreme ultraviolet (EUV) light source apparatus, and an EUV light source apparatus provided with such a nozzle protection device.
2. Description of a Related Art
Recent years, as semiconductor processes become finer, photolithography has been making rapid progress to finer fabrication. In the next generation, microfabrication of 100 nm to 70 nm, further, microfabrication of 50 nm or less will be required. Accordingly, in order to fulfill the requirement for microfabrication of 50 nm or less, for example, exposure equipment is expected to be developed by combining an EUV light source generating EUV light having a wavelength of about 13 nm and reduced projection reflective optics.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target (hereinafter, also referred to as “LPP type EUV light source apparatus”), a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity close to black body radiation can be obtained because plasma density can be considerably made larger, that the light emission of only the necessary waveband can be performed by selecting the target material, and that an extremely large collection solid angle of 2π steradian can be ensured because it is a point light source having substantially isotropic angle distribution and there is no structure surrounding the light source such as electrodes. Therefore, the LPP light source is considered to be predominant as a light source for EUV lithography, which requires power of more than several tens of watts.
Here, a principle of generating EUV light in the LPP type EUV light source apparatus will be briefly explained. By injecting a target material from a nozzle and applying a laser beam to the target material, the target material is excited and turned into plasma. Various wavelength components including extreme ultraviolet (EUV) light are radiated from thus generated plasma. Then, the EUV light is reflected and collected by using a collector mirror, which selectively reflects a desired wavelength component (e.g., a component having a wavelength of 13.5 nm) of them, and outputted to an exposure unit. For example, as the collect mirror collecting the EUV light having a wavelength near 13.5 nm, a mirror having thin films of molybdenum (Mo) and silicon (Si) which are alternately stacked on a reflecting surface is used.
The state of the target material to be supplied into the chamber has been studied variously. For the supply of the target material in a liquid state, there is a case of forming a continuous flow (target jet or continuous jet) of the target material or a case of forming a droplet-like target (droplet target). In the latter case, the droplet target is formed by a method of stirring the target material by providing vibration at a predetermined frequency to the target jet by using a vibration mechanism.
Meanwhile, in such an EUV light source apparatus, there is a problem that the nozzle for supplying the target material (target nozzle) is damaged considerably and has a short life. Although it is preferable to dispose the target nozzle in the neighborhood of an application position of the laser beam, that is, a plasma emission point in order to apply the laser beam accurately on the target material, the target nozzle is exposed to high temperature heat generated from the plasma and the temperature of the target nozzle increases extraordinarily. Further, flying particles (debris) such as fast ions or neutral particles, which are emitted from the plasma, shave components such as the target nozzle, a vibrator element, and so on by collision, and the debris attach to these components. Thereby the performances of the components are considerably deteriorated.
As a related technology, US Patent Application Publication US 2006/0043319 A1 discloses a target supply unit for the energy beam-induced generation of short-wavelength electromagnetic radiation in which a nozzle protection device is provided in the interaction chamber between the target nozzle and the plasma generation point (light emission point) (see page 1). This nozzle protection device includes a gas pressure chamber having an opening formed along a target trajectory so as not to prevent a target flow, and the gas pressure chamber is filled with buffer gas which is maintained to have a pressure of approximately several tens of millibars (see FIG. 1). This nozzle protection device prevents flying particles from the plasma from reaching the nozzle (sputter shield) by the gas filling a space through which the target material passes. Further, FIG. 3 in US 2006/0043319 A1 shows a short-wavelength electromagnetic radiation generating apparatus further provided with a heat protection plate in addition to such a sputter shield. This heat protection plate blocks heat generated from the plasma by circulation of cooling medium (heat shield).
Meanwhile, in the case of forming the target jet or the droplet target, a certain time is required until the target material injected from the target nozzle gets to have a predetermined injection pressure. Further, in this pressure increasing process (initial stage of target formation), the target material becomes spray like state, or injected intermittently, or injected from the nozzle in a direction different from a normal direction, for example, and thus, an injection state of the target material becomes unstable. In US2006/0043319A1, however, such a target formation initial stage is not taken into consideration, and there is a problem that the opening for passing the target material is blocked when the target material in the unstable state is sprayed onto the sputter shield or the heat protection plate.
From a viewpoint of protecting the target nozzle from the plasma heat, it is preferable to make the opening diameter of the heat protection plate as small as possible. Further, the target material flow including target jet or the droplet target becomes more unstable in the lower downstream. Accordingly, it is preferable to dispose the heat protection plate close to the injection outlet of the target nozzle. However, when the opening diameter of the heat protection plate is made smaller and further the heat protection plate is disposed close to the nozzle injection outlet, there arises a problem that the target material is attached and deposited onto the neighborhood of the opening at the target formation initial stage. As a result, a flow of the target material becomes disturbed, or the opening of the heat protection plate becomes blocked. On the other hand, when the opening diameter of the heat protection plate is increased for avoiding the above problem, the shield effect against the plasma heat becomes reduced. Alternatively, when the heat protection plate is disposed apart from the target nozzle, the position of the target material becomes unstable and accordingly the opening diameter has to be made larger. Also in this case, the heat shield effect will be reduced. In US 2006/0043319 A1, such instability of the target material position and a dilemma resulting therefrom are also not taken into consideration.
Japanese Patent Application Publication JP-P2002-237448A discloses an extreme ultraviolet light lithography apparatus utilizing a thin film protection coating for protecting a plurality of hardware elements disposed near a laser produced light source, from an erosion effect of energy particles which are emitted from the laser produced light source, in order to reduce an erosion effect of ion sputtering. That is, JP-P2002-237448A prevents a collector mirror from being contaminated by a sputtered material, which is generated by sputtering of a hardware surface with ions or neutral particles emitted from plasma (fire ball), by covering hardware such as a target nozzle and a target collecting tube with a diamond thin film, for example. In particular, the target nozzle is provided with an under coat of nickel (Ni) on a main body made of copper, and further a diamond thin film formed thereon, thereby increasing its strength.
Further, Japanese Patent Application Publication JP-P2003-43199A discloses a nozzle including (i) a main body having a source end portion for receiving a target material, an output end portion for injecting a spray of the target material, and a channel therebetween, and (ii) a target material transfer tube extending through the channel. This target material transfer tube includes a first end disposed close to the source end portion of the nozzle and a second end portion disposed close to the output end portion of the nozzle and having an expandable opening, in which the first end portion receives the target material and the second end portion injects the target material to the output end portion of the nozzle through the expandable opening. That is, in JP-P2003-43199A, the target material transfer tube is thermally insulated from the outside by use of a protection cap (a part of the main body) for preventing intensity reduction, which is caused by heat up of the nozzle, in the target material injected from the nozzle. In JP-P2003-43199A, the protection cap is formed of graphite, and the transfer tube is formed of stainless steel.
In JP-P2002-237448A and JP-P2003-43199A, the surface of the nozzle or the like is formed with diamond or graphite for suppressing the sputtering phenomenon caused by the flying particles from the plasma. A diamond thin film, for example, has a high thermal conductivity and an anti-sputtering property, and is surely difficult to be sputtered. However, the sputtering phenomenon cannot be perfectly prevented, and therefore, even in a small amount, sputtered particles of carbon are also generated. When such sputtered particles reach a collector mirror and are deposited on the reflection surface thereof, this reduces the reflectivity of the collector mirror. As a result, life is reduced in the collector mirror which is far more expensive than the target nozzle.
As described above, in the conventional heat shield, the instability at the target formation initial stage is not taken into consideration, and the material of the heat shield is selected only in view of extension of the nozzle life. According to the conventional technology, although the original object of the heat shield for protecting the nozzle from the flying particles such as ions emitted from the plasma and from the heat generated from the plasma could be achieved, a flow of the target material could not be realized stably, and a long-term influence (e.g., life shortening) to the peripheral components including the collector mirror could not be avoided. That is, in view of industrial application, there has been a problem that the EUV light source apparatus has a low reliability and a high running cost.